Magnetically-enhanced lattice instability in EuRh₂Si₂ under pressure

The family of ThCr₂Si₂ structured rare-earth Eu intermetallics exhibits the coexistence of three different instabilities: the competing Kondo and RKKY exchange interactions in heavy-fermion magnetic materials, as well as an isostructural collapse of the lattice particular to this type of tetragonal structure. Moreover, there is a valence instability in Eu that connects the lattice and magnetic instabilities, as the Eu³⁺ magnetic moment can vanish with an atomic volume reduction to a non-magnetic Eu²⁺ state. Here, in a series of ThCr₂Si₂ structured Eu antiferromagnets, we explore the subtle interaction and cooperation between these instabilities under pressure, using synchrotron-based Mossbauer spectroscopy and optical Raman scattering to track the evolution of the magnetism and the lattice, respectively. Exemplified by two end members of this series, we observe that magnetism disappears in EuRh₂Si₂ by 1 GPa, while it persists in EuGa₄ to beyond 36 GPa at T = 4 K. With the assistance of the magnetic instability, the structural instability in EuRh₂Si₂ demonstrates a pressure dependence of its phase line dT_s/dP nearly 100 times higher than that in EuGa₄. This dramatic contrast in behavior with applied pressure illuminates a cooperative, magnetically-enhanced structural instability.